1. Field of the Invention
This invention relates generally to imaging spectrometers and, more particularly, to a dispersive spectrometer design combined with a dualband focal plane array that extends the range of wavelengths that can be efficiently sampled for hyperspectral imaging applications.
2. Description of the Prior Art
Infrared grating spectrometers employ a grating to disperse infrared wavelengths over one or more columns of pixels to achieve separation of the wavelength elements. Because grating efficiency (the ratio of dispersed to incident radiation, within a small wavelength interval) is high over only an approximate "octave" range of wavelengths near the "blaze" wavelength (e.g., from 7 to 14 microns for an 11 micron blaze), multi-octave spectroscopy has previously required multiple grating, camera mirror, and focal plane array (FPA) "channels." Wavelength separation for the channels is achieved with optical beamsplitters (referred to as the "traditional approach" hereafter). Since higher performance, long wave infrared (LWIR) spectroscopic applications benefit from cryogenically-cooled FPAs and fore-optics, penalties associated with this traditional approach are large. For space applications in particular, the "heat lift" requirement of the space cryocooler needs to be carefully restricted, and is negatively impacted by the higher cooling requirements of the traditional approach.
Dispersive spectrometers using a prism as the dispersion element also suffer from different degradations in efficiency with wavelength, including loss of transmittance in anti-reflective coating and deviation from optimal values of angular dispersion. These limitations also limit prism spectrometers to slightly over one octave in non-degraded performance. A dual-channel grating spectrometer following the "traditional approach" is shown in FIG. 1. The fore-optics 104 collect the incident light and focus the beam at the field stop where a spectrometer slit is located. The collimator 105 forms a collimated optical beam for incidence on the beamsplitter 106 and then onto the dispersion elements 107, either a prism (shown) or a grating. The optical beamsplitter 106 can be either a wavelength or non-wavelength selective type. If wavelength selective, a dichroic reflects shorter and transmits longer infrared wavebands at near unity efficiencies. In a non-wavelength selective beamsplitter, a partially silvered mirror reflects and transmits equal amounts of the two wavebands. Two focal plane arrays 108, one for shorter and one for longer wavelengths, are required.
The invention described here achieves dual-octave (and multi-octave, as a logical extension) spectroscopy with high grating efficiency using a single dual- or multi-band FPA and grating element. The approach therefore results in compactness and simplifies the cooling requirements. Further, the traditional approach requires good alignment (both translational and rotational) among the various grating and FPA pairs for both ground and space applications. This requirement is particularly challenging to accomplish at cryogenic working temperatures.
Each of the approaches described above provide simultaneity in the collection of all wavelength elements, thereby "freezing" variations in target brightness, either intrinsic to the target, or those resulting from atmospheric or range variations. The resulting target spectrum, with wavelength elements collected simultaneously, is much less affected by these variations than are spectrometers that collect spectral data in a sequential mode.